Semiconductor processing method of providing a conductively doped layer of hemispherical grain polysilicon and a hemispherical grain polysilicon layer produced according to the method

ABSTRACT

A semiconductor processing method of providing a conductively doped layer of hemispherical grain polysilicon over a substrate includes, a) providing a layer of conductively doped silicon over the substrate to a thickness greater than about 200 Angstroms; b) depositing an undoped layer of non-polycrystalline silicon over the doped silicon layer to a thickness of from 100 Angstroms to about 400 Angstroms; c) positioning the substrate with the doped silicon and undoped non-polycrystalline silicon layers within a chemical vapor deposition reactor; d) with the substrate therein, lowering pressure within the chemical vapor deposition reactor to a first pressure at or below about 200 mTorr; e) with the substrate therein, raising pressure within the chemical vapor deposition reactor from the first pressure and flushing the reactor with a purging gas; f) with the substrate therein ceasing flow of the purging gas and lowering pressure within the chemical vapor deposition reactor to a second pressure at or below about 200 mTorr; and g) annealing the substrate having the deposited non-polycrystalline silicon layer in the presence of a conductivity enhancing impurity gas at an annealing temperature of from about 350° C. to about 600° C. and at an annealing pressure of from about 10 4  Torr to about 80 Torr to in situ both diffuse conductivity enhancing impurity into the non-polycrystalline silicon layer and transform the non-polycrystalline silicon layer into a conductively doped hemispherical grain polysilicon layer.

RELATED PATENT DATA

This patent resulted from a continuation application of U.S. applicationSer. No. 08/539,851, filed on Oct. 6, 1995 now U.S. Pat. No. 5,639,685,entitled "Semiconductor Processing Method Of Providing A ConductivelyDoped Layer of Hemispherical Grain Polysilicon And A Hemispherical GrainPolysilicon Layer Produced According To The Method" listing theinventors as John K. Zahurak, Klaus F. Schuegraf and Randhir P. S.Thakur.

TECHNICAL FIELD

This invention relates to the formation of hemispherical grainpolysilicon films in semiconductor wafer processing.

BACKGROUND OF THE INVENTION

The reduction in memory cell size required for high density dynamicrandom access memories (DRAMs) results in a corresponding decrease inthe area available for the storage node of the memory cell capacitor.Yet, design and operational parameters determine the minimum chargerequired for reliable operation of the memory cell despite decreasingcell area. Several techniques have been developed to increase the totalcharge capacity of the cell capacitor without significantly affectingthe chip area. These include structures utilizing trench and stackedcapacitors, as well as the utilization of new capacitor dielectricmaterials having higher dielectric constants.

One common material utilized for capacitor plates is conductively dopedpolysilicon. Such is utilized because of its compatibility withsubsequent high temperature processing, good thermal expansionproperties with SiO₂, and its ability to be conformally deposited overwidely varying topography.

As background, silicon occurs in crystalline and amorphous forms.Further, there are two basic types of crystalline silicon known asmonocrystalline silicon and polycrystalline silicon. Polycrystallinesilicon, polysilicon for short, is typically in situ or subsequentlyconductively. doped to render the material conductive. Monocrystallinesilicon is typically epitaxially grown from a silicon substrate. Siliconfilms deposited on dielectrics (such as SiO₂ and Si₃ N₄) result ineither an amorphous or polycrystalline phase. Specifically, it isgenerally known within the prior art that silicon deposited at wafertemperatures of less than approximately 580° C. will result in anamorphous silicon layer, whereas silicon deposited at temperatureshigher than about 580° C. will result in a polycrystalline layer. Thespecific transition temperature depends on the sourcechemicals/precursors and the reactor used for the deposition.

The prior art has recognized that capacitance of a polysilicon layer canbe increased merely by increasing the surface roughness of thepolysilicon film that is used as a capacitor storage node. Suchroughness is typically transferred to the cell dielectric and overlyingpolysilicon layer interfaces, resulting in a larger surface area for thesame planar area which is available for the capacitor. One procedureutilized to achieve surface roughening involves deposition underconditions which are intended to inherently induce a rough or ruggedupper polysilicon surface. Such include low pressure chemical vapordeposition (LPCVD) techniques. Yet, such prior art techniques areinherently unpredictable or inconsistent in the production of a ruggedpolysilicon film.

One type of polysilicon film which maximizes a roughened outer surfacearea is hemispherical grain (HSG) polysilicon, which is typicallyprovided to a thickness of from 300 Angstroms to 400 Angstroms. Such canbe deposited or grown by a number of techniques. One technique includesdirect LPCVD formation at 590° C. Another includes formation by firstdepositing an amorphous silicon film at 550° C. using diluted SiH₄ (20%)gas at 1.0 Torr, followed by a subsequent high temperaturetransformation anneal. HSG polysilicon is typically not, however, insitu doped during its deposition due to undesired reduction in grainsize in the resultant film or inherent failure of HSG to form.Accordingly, other methods are utilized to conductively dope the HSGpolysilicon after its deposition. To provide such doping, an underlayerof heavily doped polysilicon can be provided, with subsequent doping ofthe HSG polysilicon layer occurring by an annealing step to drive thedopant into the HSG layer. Alternately, dopant can be implanted into thepolysilicon after its deposition from above, although such may adverselyaffect grain shape having a tendency to smoothen the deposited HSGlayer.

One example prior art method for providing doped HSG poly is as follows.A first doped layer of polysilicon is typically deposited to serve as asubstrate for the HSG polysilicon. Then, a thin undoped amorphous layerof silicon is deposited. This layer is then subjected to an inert lowpressure atmosphere, followed by a higher pressure atmosphere in thepresence of a flowing purge gas, followed by exposure to another inertlow pressure evacuation atmosphere. Subsequently, an annealing step isconducted in an inert atmosphere to transform the undoped amorphoussilicon layer into undoped HSG polysilicon. Subsequent doping of the HSGlayer can be accomplished by out-diffusion from the underlyingpolysilicon layer if it has previously been highly conductively doped.Alternately, ion implantation can be used to conductively dope theformed HSG layer.

The prior art has attempted to provide such HSG layers as being in situconductively doped, but without much success. For example, the amorphoussilicon layer from which the HSG polysilicon is produced has beendeposited to be conductively doped. However, such dopants somehowadversely affect HSG grain growth as HSG polysilicon does not resultfrom a subsequently conducted annealing step.

Accordingly, needs remain for producing HSG layers which are in is situconductively doped during formation.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention are described below withreference to the following accompanying drawings.

FIG. 1 is a diagrammatic sectional view of a semiconductor waferfragment at one processing step in accordance with the invention.

FIG. 2 is a view of the FIG. 1 wafer fragment at a processing stepsubsequent to that shown by FIG. 1.

FIG. 3 is a view of the FIG. 1 wafer fragment at a processing stepsubsequent to that shown by FIG. 2.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

This disclosure of the invention is submitted in furtherance of theconstitutional purposes of the U.S. Patent Laws "to promote the progressof science and useful arts" (Article 1, Section 8).

In accordance with one aspect of the invention, a semiconductorprocessing method of providing a conductively doped layer ofhemispherical grain polysilicon over a substrate comprises the followingsteps:

depositing a layer of non-polycrystalline silicon over the substrate toa thickness of less than or equal to about 400 Angstroms; and

annealing the substrate having the deposited non-polycrystalline siliconlayer within a chemical vapor deposition reactor in the presence of aconductivity enhancing impurity gas under conditions effective to situboth diffuse conductivity enhancing impurity into the layer andtransform the silicon into a conductively doped hemispherical grainpolysilicon layer.

Referring to FIG. 1, a semiconductor wafer fragment in process isindicated generally with reference numeral 10. Such is shown ascomprising a bulk monocrystalline silicon substrate having anelectrically conductive diffusion region 14 provided therein. A layer 16of conductively doped silicon, preferably polysilicon or a silicide, isprovided outwardly of bulk substrate 12 over diffusion region 14. Layer16 is provided to a thickness of at least 200 Angstroms, with an examplepreferred thickness being 450 Angstroms. A layer 18 ofnon-polycrystalline silicon is conventionally deposited over substratelayer 16 to a thickness of from about 100 Angstroms to about 400Angstroms. Non-polysilicon layer 18 will typically and preferablycomprise amorphous a silicon, but might also include portions in amonocrystalline silicon phase. Ideally, layer 18 is completely undopedas deposited. Also preferably prior to deposition of layer 18, the outersurface of silicon layer 16 is subjected to a cleaning step, eitherwithin a chemical vapor deposition reactor or outwardly thereof,utilizing a dilute HF solution (i.e., 100:1) for 60 seconds. Suchremoves undesired impurities from layer 16.

Subsequently, the FIG. 1 wafer fragment 10 is prepared for an annealingstep within a chemical vapor deposition reactor. Preferably, thejust-described steps and subsequent steps are all conducted within thesame chemical vapor deposition reactor without breaking vacuum. With theFIG. 1 substrate in a chemical vapor deposition reactor to be utilizedfor an annealing step, the substrate is preferably first subjected to alowered first vacuum pressure of at or below about 200 mTorr.Thereafter, pressure within the chemical vapor deposition reactor israised from the first pressure, and the reactor is flushed with apurging gas. An example pressure within the reactor during purging is400 mTorr. Subsequently, flow of the purging gas is stopped and thereactor pressure is again lowered to some second pressure which is alsopreferably at or below about 200 mTorr. Such a treatment step serves tocleanse the reactor and wafer surface of impurities and better preparethe surface of the non-polycrystalline silicon layer for HSG formation.

Referring to FIG. 2, the wafer substrate is annealed in the presence ofa conductivity enhancing impurity gas under conditions effective to insitu both diffuse conductivity enhancing impurity into layer 18 andtransform the silicon therein into a conductively doped hemisphericalgrain polysilicon layer 20. The preferred wafer temperature during suchannealing is at least about 350° C., with a range of from 550° C. toabout 600° C. being most preferred. The preferred pressure range duringthe annealing is from about 10⁻⁴ Torr to about 80 Torr. Example gasesfor providing one of a desired p or n-type dopant impurity include PH₃,AsH₃, B₂ H₆, POCl₃ and tertiary butyl phosphine. An example flow ratefor any one of such gases during the in situ annealing treatment is from30 to 60 sccm for a 44 liter chemical vapor deposition reactor. Chemicalvapor deposition might be conducted by any of low pressure, rapidthermal, plasma-enhanced, or any combination of such techniques. Exampleannealing and in situ dopant treatment times are expected to be greaterthan five seconds for a cold-wall reactor, and greater than one minutefor a hot-wall reactor. For example, in situ treating a 300 Angstromsthick layer of non-polycrystalline silicon using a PH₃ flow rate of 60sccm in a cold-wall reactor for 10 to 20 minutes at 565° C. duringanneal is expected to result in an HSG layer having a phosphorus dopantconcentration of 10¹⁸ -10¹⁹ ions/cm³. Thus, a technique is disclosed anddescribed which includes in situ doping of an HSG layer of polysiliconduring its inherent formation.

It may also be desirable to subject such in situ doped HSG polysiliconlayer to subsequent diffusion doping. Specifically after formation, theconductivity enhancing dopant gas or other gas could continue to be fedto the reactor and the substrate temperature raised from the annealingtemperature to a higher second temperature, with an example being 750°C. Such higher temperatures will facilitate driving of conductivityenhancing impurity dopants into the previously formed and in situ dopedHSG layer.

Referring to FIG. 3, a subsequent capacitor dielectric layer 22 isdeposited to a suitable thickness typically greater than 30 Angstroms.Examples include silicon nitride, silicon dioxide, tantalum pentoxide,titanium oxide, yttrium oxide, barium strontium titanate, combinationsof these, and others. Subsequently, a conductive outer layer 24 isdeposited to an example thickness of 750 Angstroms to complete inherentformation of a capacitor construction 25. The illustrated compositelayers could of course be patterned to define the outlines of anisolated capacitor.

The above process can reduce time of processing due to in situ doping,and can enable elimination of a subsequent ion implanting step. Suchwill also result in a more uniformly doped and rougher HSG layer.

In compliance with the statute, the invention has been described inlanguage more or less specific as to structural and methodical features.It is to be understood, however, that the invention is not limited tothe specific features shown and described, since the means hereindisclosed comprise preferred forms of putting the invention into effect.The invention is, therefore, claimed in any of its forms ormodifications within the proper scope of the appended claimsappropriately interpreted in accordance with the doctrine ofequivalents.

We claim:
 1. A method of forming conductively doped hemispherical grainpolysilicon comprising annealing non-polycrystalline silicon within achemical vapor deposition reactor in the presence of a conductivityenhancing impurity gas under conditions effective to in situ bothdiffuse conductivity enhancing impurity into the silicon and transformthe silicon into conductively doped hemispherical grain polysilicon. 2.A method of forming conductively doped hemispherical grain polysiliconcomprising annealing non-polycrystalline silicon in the presence of aconductivity enhancing impurity gas under conditions effective to insitu both diffuse conductivity enhancing impurity into the silicon andtransform the silicon into a conductively doped hemispherical grainpolysilicon.
 3. A method of forming a capacitor comprising the followingsteps:depositing a layer of non-polycrystalline silicon over asubstrate; annealing the substrate having the depositednon-polycrystalline silicon layer within a chemical vapor depositionreactor in the presence of a conductivity enhancing impurity gas underconditions effective to in situ both diffuse conductivity enhancingimpurity into the layer and transform the silicon into a conductivelydoped hemispherical grain polysilicon layer; forming a dielectric layerover the hemispherical grain polysilicon layer; and forming a conductivelayer over the dielectric layer.
 4. A method of forming a capacitorcomprising the following steps:annealing non-polycrystalline silicon inthe presence of a conductivity enhancing impurity gas under conditionseffective to in situ both diffuse conductivity enhancing impurity intothe silicon and transform the silicon into a conductively dopedhemispherical grain polysilicon; forming a dielectric layer over thehemispherical grain polysilicon; and forming a conductive layer over thedielectric layer.
 5. A semiconductor processing method of providing aconductively doped layer of hemispherical grain polysilicon over asubstrate comprising the following steps:providing a layer ofconductively doped silicon over the substrate; depositing an undopedlayer of non-polycrystalline silicon over the doped silicon layer;positioning the substrate with the doped silicon and undopednon-polycrystalline silicon layers within a chemical vapor depositionreactor; with the substrate therein, lowering pressure within thechemical vapor deposition reactor to a first pressure at or below about200 mTorr; with the substrate therein lowering pressure within thechemical vapor deposition reactor to a second pressure at or below about200 mTorr; and annealing the substrate having the depositednon-polycrystalline silicon layer in the presence of a conductivityenhancing impurity gas at an annealing temperature of from about 350° C.to about 600° C. and at an annealing pressure of from about 10⁻⁴ Torr toabout 80 Torr.
 6. A semiconductor processing method of providing aconductively doped layer of hemispherical grain polysilicon over asubstrate comprising the following steps:providing a layer ofconductively doped silicon over the substrate to a thickness greaterthan about 100 Angstroms; depositing an undoped layer ofnon-polycrystalline silicon over the doped silicon layer to a thicknessof from 100 Angstroms to about 400 Angstroms; positioning the substratewith the doped silicon and undoped non-polycrystalline silicon layerswithin a chemical vapor deposition reactor; with the substrate therein,lowering pressure within the chemical vapor deposition reactor to afirst pressure at or below about 200 mTorr; with the substrate therein,raising pressure within the chemical vapor deposition reactor from thefirst pressure; with the substrate therein lowering pressure within thechemical vapor deposition reactor to a second pressure at or below about200 mTorr; and annealing the substrate having the depositednon-polycrystalline silicon layer in the presence of a conductivityenhancing impurity gas at an annealing temperature of from about 350° C.to about 600° C. and at an annealing pressure of from about 10⁻⁴ Torr toabout 80 Torr to in situ both diffuse conductivity enhancing impurityinto the non-polycrystalline silicon layer and transform thenon-polycrystalline silicon layer into a conductively dopedhemispherical grain polysilicon layer.